1. Field of the Invention
The present invention relates to a resistor-string digital/analog converter circuit and a semiconductor device including the same circuit, and more particularly to a resistor-string digital/analog converter circuit using MOS resistance as interpolation resistance, and a semiconductor device including the same circuit.
2. Description of Related Art
As one sort of digital/analog converter circuit (hereinafter, “D/A” for short), there has hitherto been known a resistor-string D/A converter circuit (R-DAC). In the resistor-string D/A converter circuit, a plurality of resistors connected in series are used and an analog voltage is extracted from a voltage dividing point corresponding to an input digital signal, whereby D/A conversion is performed. Generally, in the resistor-string digital/analog converter circuit, resolution is sometimes improved using interpolation resistance.
There has been proposed a technique by which, when a plurality of MOS resistances are used as the interpolation resistance, in order to reduce the variation in MOS resistance value ascribable to body effect, a source potential of one MOS transistor among the interpolation resistances is monitored to vary the gate potential of each MOS transistor (for example, refer to U.S. Pat. No. 5,943,000).
More specifically, the source potential (Vpol) of one MOS transistor of the interpolation resistances are monitored and the gate potential (Vgate) created based on the source potential (Vpol) monitored is supplied to the gate of all the MOS transistors. Accordingly, the technique according to the proposal reduces the variation in MOS resistance value caused by a variation in position to which the MOS transistors are connected, thereby improving linearity of D/A conversion.
However, in the technique according to the proposal, the source potential differs between each MOS transistor, and MOS resistance value is in inverse proportion to the difference between gate-source voltage (Vgs) and threshold voltage (Vth). Therefore, the MOS resistance values of the plurality of MOS transistors are not equal to each other, thus causing a problem.
That is, even when the same potential is applied to the gates of all the MOS transistors, since the source potential of each MOS transistor is different, there arises a difference in gate-source voltage (Vgs), causing nonuniformity of MOS resistance values. Consequently, satisfactory linearity of D/A conversion cannot be achieved. Particularly, when a large voltage is applied to the plurality of MOS transistors, the nonuniformity is noticeable.